Optimal component placement continues to be a surgical challenge in total hip Arthroplasty (THA), especially for the less-experienced surgeon. Dislocation and subluxation are still the most frequent early complications. Failure to achieve joint stability necessitates revision surgery in many cases, significantly increasing morbidity for the patient and cost to the healthcare system.
Dislocation and subluxation have been well correlated to implant impingement caused by malpositioning of components. Angular orientation in terms of anteversion/antetorsion and inclination has been identified as the related geometric key factor. Excessive anteversion or inclination is one of the most common surgical errors resulting in dislocation. It is known that there is a statistically significant relationship between the increased acetabular component anteversion and the increased anterior dislocations. Furthermore, the risk of dislocation is significantly higher in those who have already experienced dislocation or after revision surgery. As the patients undergoing revision surgery will have more health problems, obtaining proper cup positioning during the primary surgery is crucial.
The optimal ranges of anteversion and inclination of the acetabular component have been extensively debated in the literature. Several so-called “safe zone” have been suggested. Lewinnek et al. (Lewinnek G E, Lewis J L et al: Dislocation after total hip-replacement arthroplasties. J Bone Joing Surg 60A:217, 1978) describes a safe zone of 5° to 25° for anteversion and 30° to 50° for inclination. They found that acetabular cups placed outside this safe zone were approximately four times as likely to dislocate. In a prospective study of 441 THAs, McCollum and Gray determined that the safe range for cup position was 30°-50° abduction and 20°-40° flexion (McCollum D E, Gray W J G: Dislocation after Total Hip Arthroplasty. Causes and Prevention. Clin Orthop 261:159-70, 1990).
Optimal cup positioning requires that the surgeon attains adequate anteversion and inclination of the acetabular component with respect to each individual pelvis morphology in a reproducible basis. The intra-operative mechanical guides, though easy to use, does not respect the individual pelvis morphology and utilize the plane of the operating room table as a reference when orienting the cup. This results in hips that are implanted outside the safe zone defined by Lewinnek et al. using only the mechanical guidance.
Recently, several groups (Asayama I., et al.: Intraoperative pelvic motion in total hip Arthroplasty. J. Arthroplasty, 19(8): 992-997, 2004; Ezoe M., et al.: Pelvic motion during total hip Arthroplasty with translateral and posterolateral approaches. J Orthop Sci 10: 167-172, 2005; Wilairatana V. and Prasongchin P.: Acetabular position setting in total hip Arthroplasty by using V-Inclinometer. J Med Assoc Thai, 87(4): 353-355, 2004; Echeverri S., et al.: Reliable acetabular cup orientation with a new gravity-assisted guidance system. J. Arthroplasty 21(3): 413-419, 2006; Echeverri S.: Orientation device for surgical implement. International Patent Application Number: PCT/CH2004/000466) described methods to use the constant direction of the force of gravity as a reference in THA. Asayama et al. introduced a 3-direction indicator to control intra-operative pelvic motion during THAs. The 3-direction indicator incorporates a digital compass with 2 goniometers, as well as a pendulum and target apparatus. It allows for controlling pelvic motion by measuring the three-dimensional (3D) angle formed by the gravitational direction and the Steinmann pin inserted into the iliac bone to fix the direction indicator. Using a pendulum and a goniometer, Wilairatana and Prasonchin proposed a so-called V-Inclinometer to measure the pelvic inclination. Different from these systems, Echeverri et al. described a gravity-assisted navigation system (GANS) to control both the pelvic motion and the acetabular component placement. Their system requires two bull's-eye bubble levels with one designed for controlling the pelvic motion in strict lateral decubitus and the other for controlling cup orientation for targeted anteversion and inclination. Like any other mechanical guide, this system is simple to use, but it is also highly flawed. This is due to the fact that the alignment system developed by Echeverri et al. was calibrated only with respect to one pelvis without considering the morphological difference between the future pelvis to be navigated and the pelvis used for calibration. It simply does not work due to the inherent morphological variations in human being. A recent simulation study of this method on 48 patient data revealed a maximum anteversion error of as high as 15° (Dong X, Echeverri S, Nolte L-P, Vallotton J, and Zheng G: Acetabular cup orientation using a statistical data based calibration table. In proceedings of the 8th Annual Meeting of the International Society for Computer Assisted Orthopaedic Surgery, Hongkong, China, Jun. 4-7, 2008. PP. 262-265).
In principle, computer-assisted THA Systems such as CT-based computer aided THA, fluoroscopy-based computer aided THA or image-free navigation allow for better component placement. However, due to the costs of the equipment as well as its service and maintenance, these techniques are very expensive. Furthermore, these systems are very complex and require additional training for the users.